The entry of calcium into cells through voltage-gated calcium channels mediates a wide variety of cellular and physiological responses, including excitation-contraction coupling, hormone secretion and gene expression (Miller, R. J., Science (1987) 235:46-52; Augustine, G. J. et al., Annu Rev Neurosci (1987) 10: 633-693). In neurons, calcium channels directly affect membrane potential and contribute to electrical properties such as excitability, repetitive firing patterns and pacemaker activity. Calcium entry further affects neuronal functions by directly regulating calcium-dependent ion channels and modulating the activity of calcium-dependent enzymes such as protein kinase C and calmodulin-dependent protein kinase II. An increase in calcium concentration at the presynaptic nerve terminal triggers the release of neurotransmitter, which also affects neurite outgrowth and growth cone migration in developing neurons.
Calcium channels mediate a variety of normal physiological functions, and are also implicated in a number of human disorders. Examples of calcium-mediated human disorders include but are not limited to congenital migraine, cerebellar ataxia, angina, epilepsy, hypertension, ischemia, and some arrhythmias. The clinical treatment of some of these disorders has been aided by the development of therapeutic calcium channel antagonists (e.g., dihydropyridines, phenylalkyl amines, and benzothiazapines all target L-type calcium channels) (Janis, R. J. & Triggle, D. J., Ion Calcium Channels: Their Properties, Functions, Regulation and Clinical Relevance (1991) CRC Press, London).
Native calcium channels have been classified by their electrophysiological and pharmacological properties into T-, L-, N-, P/Q- and R-types (reviewed in Catterall, W., Annu Rev Cell Dev Biol (2000) 16: 521-555; Huguenard, J. R., Annu Rev Physiol (1996) 58: 329-348). T-type (or low voltage-activated) channels describe a broad class of molecules that transiently activate at negative potentials and are highly sensitive to changes in resting potential.
The L-, N- and P/Q-type channels activate at more positive potentials (high voltage-activated) and display diverse kinetics and voltage-dependent properties (Catterall (2000); Huguenard (1996)). T-type channels can be distinguished by having a more negative range of activation and inactivation, rapid inactivation, slow deactivation, and smaller single-channel conductances. There are three subtypes of T-type calcium channels that have been molecularly, pharmacologically, and elecrophysiologically identified: these subtypes have been termed α1G, α1H, and α1I (alternately called Cav 3.1, Cav 3.2 and Cav 3.3 respectively).
T-type calcium channels are involved in various medical conditions. In mice lacking the gene expressing the α1G subunit, resistance to absence seizures was observed (Kim, C. et al., Mol Cell Neurosci (2001) 18(2): 235-245). Other studies have also implicated the a1H subunit in the development of epilepsy (Su, H. et al., J Neurosci (2002) 22: 3645-3655). There is strong evidence that some existing anticonvulsant drugs, such as ethosuximide, function through the blockade of T-type channels (Gomora, J. C., et al., Mol Pharmacol (2001) 60: 1121-1132).
Low voltage-activated calcium channels are highly expressed in tissues of the cardiovascular system. Mibefradil, a calcium channel blocker 10-30 fold selective for T-type over L-type channels, was approved for use in hypertension and angina. It was withdrawn from the market shortly after launch due to interactions with other drugs (Heady, T. N., et al., Jpn J Pharmacol. (2001) 85:339-350).
There is also a growing body of evidence that suggests that T-type calcium channels are abnormally expressed in cancerous cells and that blockade of these channels may reduce cell proliferation in addition to inducing apoptosis. Recent studies also show that the expression of T-type calcium channels in breast cancer cells is proliferation state dependent, i.e. the channels are expressed at higher levels during the fast-replication period, and once the cells are in a non-proliferation state, expression of this channel is minimal. Therefore, selectively blocking calcium channel entry into cancerous cells may be a valuable approach for preventing tumor growth (PCT Patent Application Nos. WO 05/086971 and WO 05/77082; Taylor, J. T., et al., World J. Gastroenterol (2008) 14(32): 4984-4991; Heo, J. H., et al., Biorganic & Medicinal Chemistry Letters (2008) 18:3899-3901).
Growing evidence suggests T-type calcium channels are also involved in pain (see for example: US Patent Application No. 2003/086980; PCT Patent Application Nos. WO 03/007953 and WO 04/000311). Both mibefradil and ethosuximide have shown anti-hyperalgesic activity in the spinal nerve ligation model of neuropathic pain in rats (Dogrul, A., et al., Pain (2003) 105:159-168). In addition to cardiovascular disease, epilepsy (see also US Patent Application No. 2006/025397), cancer and chronic and acute pain, T-type calcium channels have been implicated in diabetes (US Patent Application No. 2003/125269), sleep disorders (US Patent Application No. 2006/003985), Parkinson's disease (US Patent Application No. 2003/087799); psychosis such as schizophrenia (US Patent Application No. 2003/087799), overactive bladder (Sui, G.-P., et al., British Journal of Urology International (2007) 99(2): 436-441; see also US 2004/197825), renal disease (Hayashi, K., et al., Journal of Pharmacological Sciences (2005) 99: 221-227), neuroprotection and male birth control.
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